This invention relates to liquid crystal (LC) devices and, more particularly, to such devices which exhibit the flexoelectric effect.
The flexoelectric effect is exhibited by LC molecules which form structures that have polar symmetry; for example, geometric configurations characterized by splay (FIGS. 1-2) and/or bend (FIGS. 3-4) deformations. The effect has two forms, similar to the piezoelectric effect. Either an electric polarization P induces splay or bend curvature, or, vice versa, the curvature induces an electric polarization. this invention relates to the first form; an applied electric field E induces polarization P which in turn induces curvature (FIGS. 1-4). Reversing the sign of the electric field likewise reverses the direction of P and of the curvature (FIG. 1 vs. FIG. 2; FIG. 3 vs. FIG. 4) since the two are related in a polar fashion.
An applied electric field also induces parallel molecular alignment due to the coupling to the dielectric anisotropy. If the dielectric anisotropy is too large, the flexoelectric effect induced by an applied electric field may not be observed.
The first proposal of the flexoelectric effect was made by R. B. Meyer in Phy. Rev. Lett., Vol. 22, p. 918 (1969). In an ordinary nematic LC material, which is initially uniformly aligned, he proposed that a uniform electric field would induce the formation of a continuously rotating (not in time, but in space) director structure having alternating bands of splay and bend deformation of the type shown in FIG. 2 of the article. However, the formation of such a structure from a uniformly aligned nematic has never been observed for two reasons. First, the flexoelectric effect must compete with ordinary dielectric anisotropy, and the latter coupling usually dominates, maintaining uniform orientation (i.e., molecules lined up with E). Second, the continuously rotating director pattern would have to be formed by the generation of a periodic array of disclination line defects near the electrode surfaces and at the centers of the splay bands. However, these defects would occur most easily with high static fields which undesirably may induce electrohydrodynamic instabilities that tend to obscure the splay-bend structure.
On the other hand, the only proposal of the flexoelectric effect in cholesteric or chiral nematic LC materials is found in U.S. Pat. No. 4,564,266 granted to G. E. A. Durand on Jan. 14, 1986. However, in Durand the molecules are oriented so as to form a curved segment 3 which may be viewed as having an axis along the Oy direction parallel to the plates 1 and 2 (FIG. 2) but perpendicular to the electrodes on the front and rear walls of cell 10 (col. 8, lines 40-43). Consequently, the axis is also perpendicular to the direction of light L and parallel to the direction of the electric field E. In addition, Durand's LC material does not contain periodic bands of splay and bend.